Olefin polymerizations are frequently carried out using monomer, diluent and catalyst and optionally co-monomers and hydrogen in a reactor. When the polymerization is performed under slurry conditions, the product consists usually of solid particles and is in suspension in a diluent. The slurry contents of the reactor are circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. The product is discharged by means of settling legs, which operate on a batch principle to recover the product. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry.
Alternatively, the product slurry may be fed to a second reactor serially connected to the first reactor where a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product is a bimodal polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, and has a bimodal molecular weight distribution. The resultant product will also usually consist of solid particles in suspension in a diluent and will then be discharged from the second reactor using settling legs in a similar way as explained above.
The product slurry recovered in an olefin polymerization process comprises a slurry of polymer solids in a liquid that contains diluent, dissolved unreacted monomer, and optionally dissolved unreacted co-monomer. Typically this liquid also includes traces of heavier elements, e.g. oligomers, and lighter components including H2, N2, O2, CO and/or CO2. Catalyst will generally be contained in the polymer.
Once recovered from the reactor, the product slurry is discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers and optionally unreacted co-monomers are flashed off. Afterwards, it is highly desirable to further treat the vapors in order to recover the unreacted monomer, optionally unreacted co-monomer and the diluent, since there is an economic interest in re-using these separated components including the monomer, co-monomer, and the diluent, in a polymerization process.
It is known in the art that a vaporous stream comprising unreacted monomer, unreacted co-monomer and diluent issued from the effluent of a polymerization process may be treated in a distillation system for separation of its components. Traditionally, the diluent is captured through a complicated process so that such diluent can be recycled to the reactor.
U.S. Pat. No. 4,589,957 for instance describes a separation process of a hydrocarbon-containing vaporous stream comprising monomer, co-monomer and diluent issued from the effluent of a homo-polymerization and/or co-polymerization process. The described process comprises subjecting the vaporous stream to a two-stage distillation provided with a common accumulation zone wherein the condensate from the accumulation zone serves as the source of feed for the second distillation and reflux for the first distillation.
However, a problem encountered in many distillation systems, is that there is a sub-optimal separation of lighter components, including H2, N2, O2, CO and/or CO2, from recovered diluent. As a consequence, use of separated diluent streams containing these components in a polymerization process may seriously reduce polymerization efficiency and result in sub-optimal polymerization conditions. Especially in the case of re-using separated diluent streams in a polymerization process for preparing bimodal polymer product, it is for instance required to recover diluent streams wherein the residual amount of lighter components such as hydrogen, is substantially reduced in order to be able to use these diluent streams in reactors wherein the higher molecular weight component of a bimodal polymer product is prepared.
An example of a recovery process that is currently applied to meet this requirement involves the production of large amounts of diluent streams that are substantially free of olefin. However, such recovery process involves the re-utilisation of a diluent stream which is in fact too pure for that purpose as it substantially lacks olefin monomer and hence also is too costly for that use. Moreover, separation methods adapted to recover large amounts of substantially olefin-free diluent entail a number of problems and disadvantages, including inter ails requiring high amounts of energy for carrying out the separation process; resulting in increased amounts of olefin monomers that have to be separated from lighter components such as those given above; increased loss of olefin monomer, reduced stability of distillation systems; etc.
In view of the above, there remains a great need in the art for optimised methods for recycling hydrocarbon-containing feed streams that need to be separated into streams that can be recycled to a polymerization process, especially wherein bimodal polyolefins, such as for instance bimodal polyethylene, is prepared. Furthermore, there is a need in the art to provide a diluent recycle process that is less expensive to construct and/or to operate.
Summary The Applicants provide a process that overcomes at least some of the above-mentioned problems. Thereto an optimised process for separating a hydrocarbon-containing feed stream into different product streams and for re-using said separated product streams is provided. More in particular, the herein provided process permits to optimally recycle the separated streams in a polymerization process for preparing bimodal polymer.
In a first aspect, the invention thereto provides a process for recycling product streams separated from a hydrocarbon-containing feed stream comprising olefin monomer, one or more optional olefin co-monomer, hydrocarbon diluent and components such as H2, N2, O2, CO, CO2, and formaldehyde, wherein said hydrocarbon-containing feed stream is separated by the steps of:                a) introducing said feed stream into a first distillation column for subjecting said feed to distillation conditions adapted to remove                    a1) a bottom stream comprising hydrocarbon diluent and one or more optional co-monomer, and            a2) an overhead stream comprising hydrocarbon diluent, olefin monomer and components such as H2, N2, O2, CO, CO2, and formaldehyde;                        b) condensing the overhead stream issued from the first distillation column in step a2) to form a condensate and storing said condensate in a separator (108) adapted to separate a vapor stream and a liquid stream;        c) removing from said separator said vapor stream comprising olefin monomer, hydrocarbon diluent and components such as formaldehyde, H2, N2, O2, CO and CO2;        d) condensing the vapor stream removed in step c) to form a condensate and storing said condensate in a separator adapted to separate a vapor stream and a liquid stream (15);        e) removing from said separator said liquid stream of step d);        f) separating said liquid stream into a first side stream comprising hydrocarbon diluent and olefin monomer; and a remainder stream;        g) introducing said remainder stream in a second distillation column and subjecting said remainder stream to distillation conditions adapted to remove                    g1) a bottom stream comprising substantially olefin-free hydrocarbon diluent,            g2) a substantially hydrogen-free second side stream comprising hydrocarbon diluent and olefin monomer, and            g3) an overhead vapor stream comprising olefin monomer, hydrocarbon diluent and components such as formaldehyde, H2, N2, O2, CO and CO2.                        
In accordance with the present method, it is further noted that vapor stream issued from the separator of the first distillation column is sent/fed to the overhead condenser of the second distillation column. The present invention is thus characterized in that it comprises at least two condensation/separation cycles provided in series. This advantageously permits to limit monomer loss, and therefore to reduce production costs. In particular, in the case of polyethylene production, such mode of operation allows to limit the loss of ethylene with the ethane purge in case of bimodal configuration of the reactors. In another embodiment of the present process, a portion of the condensate stored in step b) is removed as liquid stream and passed as reflux to the first distillation column.
In a preferred embodiment, the invention provides a process wherein said first and said second side streams are recycled in a polymerization process for preparing bimodal polyolefin comprising at least two different polyolefin fractions that have been obtained in two different polymerisation reactors connected to each other in series, and wherein one of said fractions has a higher molecular weight than said other fraction, and wherein said second side stream is re-used in the polymerization process wherein the polyolefin fraction having the higher molecular weight is prepared, and wherein said first side stream is re-used in the polymerization process wherein the other polyolefin fraction is prepared.
In other words, the present process involves the steps of recycling said first and said second side streams in a polymerization process for preparing bimodal polyolefin comprising at least two different polyolefin fractions that have been obtained in two different polymerisation reactors connected to each other in series, and wherein one of said fractions has a higher molecular weight than said other fraction. The second side stream is re-used in the polymerization process wherein the polyolefin fraction having the higher molecular weight is prepared and the first side stream is re-used in the polymerization process wherein the other polyolefin fraction, i.e. the polyolefin fraction having the lower molecular weight, is prepared. The first side stream can thus be fed to a reactor in which the polyolefin fraction having the higher molecular weight is prepared, while the second side stream can be fed to the reactor in which said other polyolefin fraction is prepared.
In yet another embodiment, the invention provides a process in which said bottom stream of step g1) is re-used in a polymerization process for preparing bimodal polyolefin comprising at least two different polyolefin fractions that have been obtained in two different polymerisation reactors connected to each other in series, and wherein one of said fractions has a higher molecular weight than said other fraction, and wherein said bottom stream is re-used in the polymerization process wherein the polyolefin fraction having the higher molecular weight is prepared.
In another embodiment, the invention provides a process comprising the steps of                h) condensing the overhead vapor stream obtained in step g3), optionally in admixture of the vapor stream removed in step c) to form a condensate, and storing the condensate thus formed in a separator; and        i) subjecting the stored condensate obtained in step h) to steps e) to g).        
Preferably, the process further comprises the step of removing from the condensate stored in step d) a vapor stream comprising olefin monomer, and components such as formaldehyde, H2, N2, O2, CO and CO2; and recovering olefin monomer from said vapor stream.
In yet another embodiment, the present invention provides a process which may further comprise the step of introducing the bottom stream of step al) in a third distillation column (3) for subjecting said bottom stream to distillation conditions adapted to remove 1) a side stream comprising one or more optional co-monomer, 2) an overhead stream comprising hydrocarbon diluent and optionally co-monomer, and 3) a bottom stream comprising heavy components.
Preferably the side stream 1) is taken from the lower part of the distillation column e.g. from tray 3 of the column, when counting from the bottom of the column. Generally, the overhead stream of the third distillation column will contain only minor amounts of co-monomer.
This overhead stream of the third distillation column comprising hydrocarbon diluent can be fed back to the first distillation column. Preferably, the overhead stream which exits the top of the third distillation column is first cooled down in an overhead condenser of the third distillation column. Then the condensed stream, issued at the outlet of the condenser of the third distillation column is collected in a reflux drum of the third distillation column. The condensate can then be split up in two parts: a first part thereof is sent as reflux to the third distillation column and a second part is recycled as feed to the first distillation column.
Alternatively or in combination therewith, bottom stream of step a1) can also advantageously be re-used in the polymerization process for preparing bimodal polyolefin. Especially it can be fed to the reactor in which the polyolefin fraction with the higher molecular weight fraction is prepared. In bimodal configuration, the reactor in which the polyolefin fraction with the higher molecular weight fraction is prepared is also the one in which co-monomer (e.g. hexene) concentration is the highest.
It shall be noted that all values that are given herein in ppm are meant to refer to values of ppm by weight. Hence the terms “ppm” and “ppm by weight” are used herein as synonyms.
In another embodiment, the present invention provides a process wherein said first side stream of step f) comprises at least 3wt %, more preferably at least 5 wt % olefin monomer.
In another embodiment, the present invention provides a process wherein said first side stream of step f) comprises less than 10 wt % olefin monomer, and for instance less than 8wt % olefin monomer.
In another embodiment, the present invention provides a process wherein said first side stream of step f) comprises between 100 and 10000 ppm by weight hydrogen, preferably between 100 to 5000 ppm by weight. In a preferred embodiment said first side stream of step f) comprises less than 500 ppm by weight hydrogen.
In still another embodiment, the invention provides a process wherein said second side stream of step g2) is removed from the upper third of said second distillation column. In an embodiment, the present invention provides a process wherein said second side stream of step g2) comprises at least 2 wt % olefin monomer, preferably at least 2.5 wt %, and more preferably at least 3 wt % olefin monomer. In another embodiment, the present invention provides a process wherein said second side stream of step g2) comprises at most 5 ppm by weight, preferably at most 2 ppm by weight, more preferably at most 1 ppm by weight hydrogen.
In yet another embodiment, the present invention provides a process wherein said bottom stream of step g1) comprises less than 5 ppm by weight of olefin monomer, and preferably less than 1 ppm by weight of olefin monomer. In another embodiment, the present invention provides a process wherein said bottom stream of step g1) comprises less than 5000 ppm by weight of olefin co-monomer.
In another preferred embodiment, a process is provided wherein said hydrocarbon-containing feed stream comprising olefin monomer, co-monomer and hydrocarbon diluent is an effluent stream obtained from a polymerization process for preparing monomodal or bimodal polyolefin. Preferably, said olefin monomer is ethylene, said co-monomer is 1-hexene and said hydrocarbon diluent is isobutane.
The present process includes the recovery of a side stream which is substantially free of hydrogen. This side stream is as rich as possible in olefin monomer while still poor enough in hydrogen, so that it can be fed to the reactor wherein the polymer fraction having the higher molecular weight is prepared during a bimodal polymerisation process. Consequently, the need for using an olefin-free hydrocarbon stream for that same purpose is strongly reduced. Since the flow rate of steam used for reboiling the distillation column is directly proportional to the flow rate of olefin-free hydrocarbon stream to be obtained as bottom stream, reducing this bottom stream further makes it possible to significantly reduce the steam consumption necessary to ensure proper re-boiling of the distillation column.
Another advantage of the present invention is that said substantially hydrogen-free side stream will take up olefin monomer, and hence significantly reduce—e.g. by more than 50%—the incondensable vaporous stream which contains the main part of the hydrogen entering the recycle section and which is removed from the recycle section and sent to a monomer recovery unit. Hence, the present invention permits to recover a larger portion of the olefin monomer entering the recycle section before it is sent to a recovery unit compared to currently applied recovery processes. For instance, the present invention permits to recover a larger portion of ethylene monomer entering the recycle section before it is sent to an ethylene recovery unit (ERU) compared to currently applied recovery processes. In accordance with the present process, recovering a substantially hydrogen-free side stream from the hydrocarbon feed stream permits to significantly reduce the loss of ethylene monomer.
Further in accordance with the present process less incondensable vaporous stream containing lighter components needs to be sent to the monomer recovery unit and the size of the monomer recovery unit, for instance an ethylene recovery unit, can be significantly reduced. This has important economic and environmental repercussions and benefits.
Furthermore, unexpectedly, the Applicants have seen that the present process involving the recovery of a) a first side stream comprising hydrocarbon diluent and olefin monomer; b) a second side stream which is substantially hydrogen-free and comprises hydrocarbon diluent and olefin monomer, c) a bottom stream comprising substantially olefin-free hydrocarbon diluent, and d) an overhead vapor stream comprising remaining olefin monomer, remaining hydrocarbon diluent and remaining components such as formaldehyde, H2, N2, O2, CO and CO2 has improved efficiency and stability compared to conventional distillation system which lack recovery of said second side stream.
The present optimized recovery process is particularly suitable for providing diluents streams for re-use in a polymerization system for preparing bimodal polymer product. In particular, the present invention provides a process enabling to separately recover I) a substantially hydrogen-free diluent side stream that can be used in the reactor wherein the higher molecular weight fraction of a bimodal polymer is prepared; and II) a diluent side stream that can be used in the reactor wherein the lower molecular weight fraction of a bimodal polymer is prepared. Thus the present process optimally provides two side streams that each can be re-used to feed diluent to the respective reactors applied in the polymerization process for preparing bimodal polyolefins, for instance bimodal polyethylene.
In another aspect, the invention therefore also relates to the use of a process according to the invention in a polymerization process for preparing bimodal polyolefin, such as for instance bimodal polyethylene, comprising at least two different polyolefin fractions that have been obtained in two different polymerisation reactors connected to each other in series, and wherein one of said fractions has a higher molecular weight, comprising the steps of:                feeding olefin monomer, a diluent, at least one polymerization catalyst, optionally hydrogen, and one or more optional olefin co-monomer(s) to a first reactor,        polymerizing said olefin monomer in said first reactor to produce a polymer slurry comprising a first polyolefin fraction in the diluent,        transferring said polymer slurry from said first reactor to a second reactor,        feeding olefin monomer, a diluent, optionally hydrogen, and one or more optional olefin co-monomer(s) to said second reactor,        polymerizing said olefin monomer and said one or more optional olefin co-monomer(s) in said second reactor to produce a slurry comprising a second polyolefin fraction in the diluent, said second polyolefin fraction having a different molecular weight than the polyolefin fraction produced in said first reactor, and        discharging from said second reactor a slurry comprising bimodal polyolefin in said diluent,        recovering bimodal polyolefin from the slurry by separating at least a majority of the diluent from the slurry in a hydrocarbon-containing feed stream, and        subjecting said hydrocarbon-containing feed stream to a process as described herein.        
In yet another aspect, the invention also provides a polymerization system for preparing bimodal polyolefin comprising two polymerisation reactors connected to each other in series operably connected to a distillation system, the distillation system comprising a first distillation column and a second distillation column which are operably connected to each other in series; and
wherein said first distillation column which is configured to separate a hydrocarbon-containing feed stream comprising olefin monomer, optionally one or more co-monomer and hydrocarbon diluent, into
                a bottom stream comprising hydrocarbon diluent and one or more optional co-monomer, and        an overhead stream comprising hydrocarbon diluent, olefin monomer and components such as H2, N2, O2, CO, CO2, and formaldehyde;is provided with at least one condenser for condensing said overhead feed stream to form a condensed stream and at least one separator, operably connected to said condenser and adapted to separate said condensed stream in a vapor stream and a liquid stream; andwherein said second distillation column which is configured to separate a hydrocarbon-containing feed stream comprising olefin monomer, optionally one or more co-monomer and hydrocarbon diluent, into        a bottom stream comprising substantially olefin-free hydrocarbon diluent,        a substantially hydrogen-free side stream comprising hydrocarbon diluent and olefin monomer, and        an overhead stream comprising olefin monomer, hydrocarbon diluent and components such as formaldehyde, H2, N2, O2, CO and CO2,is provided with at least one condenser for condensing said overhead stream to form a condensed stream and at least one separator, operably connected to said condenser and adapted to separate said condensed stream in a vapor stream and a liquid stream, of which a part is separated as a first side stream comprising hydrocarbon diluent and olefin monomer.        
The present invention will be further disclosed in detail hereunder. The description is only given by way of example and does not limit the invention. The reference numbers relate to the hereto-annexed figures.